I'm eager to find out if Call of Duty works well enough on the mini to play regularly.... anyone know?

judging by the system requirements of Call of Duty (Mac OS X:10.2.8,
CPU:G4@867MHz, RAM:256MB, 32MB VRAM) it should play no problem. but it may be best to download a demo just to make sure. OS, CPU and RAM are all good with the mini just the VRAM that is at minimum.

I've done some gaming on my Mini and a 20" 2 Ghz G5 iMac. It's not too bad with older games, but any newer FPS games or ones that require decent horsepower just don't work too well. You can try turning down graphics options & details, but it just makes most of them look pretty bad.

For example, I've tried playing America's Army on our 2Ghz G5 iMac (and also the mini). I had to turn all of the settings down to ultra low and turn off any other 3D "frills" just to get above 10 FPS (still dropped below 10 in firefights). The result is an image so poor it looks like a bad VHS copy of an amateur cartoon.

The result is that I keep a PC for gaming. By contrast I can maintain 60 FPS or better and have all settings at their maximum. The game designers are coming up with some amazingly detailed graphics these days and I'm simply not willing to give up viewing them.

That said, the Mini is a great little system and if you just need to get a "gaming fix" from time to time, there is definitely some fun to be had.